Dual-bed catalytic distillation tower and method for preparing dimethyl ether using the same

ABSTRACT

A dual-bed catalytic distillation tower has a catalytic column having an upper catalytic bed filled with low temperature dehydration catalysts and a lower catalytic bed filled with high temperature dehydration catalysts. When using the dual-bed catalytic distillation tower, the feed may be fed to the tower at the top of the upper catalytic bed, between the upper and lower catalytic beds, or at the bottom of the lower catalytic bed for dehydration to obtain DME. The dual-bed catalytic distillation tower has the advantage of flexible set up depending on various feeds, such as anhydrous or crude methanol and on different grades of DME to be obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a catalytic distillation tower used for preparing dimethyl ether (DME), especially a dual-bed catalytic distillation tower. The present invention also relates to a method for preparing dimethyl ether using the same.

2. Description of the Prior Arts

Dimethyl ether (DME) is prepared by using different raw materials such as coal, nature gas, petroleum coke and biochar. The methanol is firstly prepared from the raw materials, and the methanol is dehydrated to obtain DME. In the dehydration reaction, catalysts are needed to carry out the reaction. Solid-state acid catalysts such as zeolite, silicone, aluminum oxide, resin etc. (Spivey, J. J., 1991) or modified acid ion exchange resin (U.S. Pat. No. 6,740,783) are commonly used in the dehydration. Resin catalysts are low temperature catalysts with reaction temperatures ranging from 70 to 150° C. and almost 100% conversion of methanol can be obtained. However, the low temperature catalysts cannot bear high temperature. Other catalysts are high temperature catalysts with reaction temperatures ranging from 200 to 350° C. When the reaction temperature is getting higher, however, methanol and DME will be further dehydrated to C2-C4 olefin, thereby resulting in the loss of catalyst activity and reduce of DME yield.

Further, to reduce the energy consumption during the catalytic distillation process, the catalysts are put in the catalytic distillation tower so the heat generated from the methanol dehydration reaction can be used for catalytic distillation. The requirement for this catalytic distillation process is that the catalytic reaction and distillation can happen at the similar tower pressure and temperature range.

US 2007/0066855 A1 discloses a method for production of DME using catalytic distillation tower so the heat generated from dehydration can be fully used and reduce the production cost. However, the tower temperature and pressure used for dehydration reaction result in that methanol and water in the stripping column cannot be separated easily. FIG. 1 shows the relative volatility of methanol and water at different tower pressures and can be used to calculate and estimate the difficulty for separating methanol and water at high tower pressure. FIG. 1 shows that the relative volatility of methanol and water reaches 1 (i.e. the azeotropic zone) when the mole fraction of liquid water equals 0.825 at a tower pressure of 18 bar. Azeotrpe composition of methanol and water results in separation difficulties.

WO 2007/014534 provides high temperature dehydration in the catalytic distillation tower by using high temperature dehydration catalysts at a reaction temperature ranging from 160 to 180° C. and under a tower pressure ranging from 18 to 23 bar. However, the energy consumption of the condenser and reboiler increases in order to drive the catalytic reaction under high temperatures. U.S. Pat. No. 5,684,213 mentions high temperature dehydration reaction at a reaction temperature ranging from 350 to 400° C. and under 600 psi of tower pressure. When dehydration is carried out at such a high temperature, olefin side products increase to cause the decay of catalyst activity, resulting in lower DME yield. Hydrogen is added to the catalytic distillation tower to inhibit undesired side products generated. However, adding hydrogen increases the production cost and the reaction complexity.

The technique of using low temperature dehydration catalysts in the catalytic distillation tower is also disclosed. For example, WO 2007/014534 provides that dehydration reaction is carried out at a reaction temperature ranging from 130 to 158° C. and under a tower pressure ranging from 10 to 18 bar by using low temperature acid ion exchange catalysts. However, the low temperature catalysts can only be used at temperature lower than 140° C. When the reaction is carried out at a low temperature and pressure, the dew point of high purity DME at the top of the tower will decrease. For example, the dew point of DME becomes 40.3° C. when the tower pressure is as low as 9 bar and therefore using 20° C. industrial water to condense DME will be impossible. When the tower pressure is adjusted to 12 bar, the temperature at the catalytic column will be higher than 150° C., and the low temperature catalysts will be damaged.

All the above-mentioned methods use single-bed catalytic system in the catalytic distillation tower, i.e. only high temperature or low temperature dehydration reaction is carried out. In the conventional methods, the catalytic distillation tower cannot be easily adjusted depending on the grades of the feeding methanol and of the obtained DME because none of the conventional methods can provide suitable and complementary tower temperature and pressure conditions.

SUMMARY OF THE INVENTION

Therefore, the purpose of the present invention is to provide a catalytic distillation method for preparing dimethyl ether (DME) in which the tower temperature and pressure can be properly adjusted depending on the relation among DME, methanol and water in the catalytic distillation and the dehydration catalysts selected.

Specifically, the present invention relates to a dual-bed catalytic distillation tower and the method for preparing DME using the same.

The present invention provides a dual-bed catalytic distillation tower comprising a catalytic column. The catalytic column comprises an upper catalytic bed and a lower catalytic bed. The upper catalytic bed is filled with low temperature dehydration catalysts and the lower catalytic bed is filled with high temperature dehydration catalysts. At least one feed port is located at the top of the upper catalytic bed, between the upper catalytic bed and the lower catalytic bed, and at the bottom of the lower catalytic bed.

Preferably, the low temperature dehydration catalyst is Amberlyst® 15 acid ion exchange resin or Amberlyst® 35 acid ion exchange resin.

Preferably, the high temperature dehydration catalyst is selected from fluorinated transition metal oxide, sulfate transition metal oxide, β zeolite and HZSM-5. All high temperature dehydration catalysts are covered by Teflon to enhance hydrophobic property of the catalysts.

The present invention also provides a method for preparing dimethyl ether by using aforesaid dual-bed catalytic distillation tower wherein the catalytic column is used for dehydration of methanol. The feeding stream containing methanol is fed from the feed port located at the top of the upper catalytic bed, between the upper catalytic bed and the lower catalytic bed, or at the bottom of the lower catalytic bed at the tower pressure ranging from 6 to 30 bar for dehydration to obtain DME.

The temperature of the catalytic distillation tower of the present invention is at the range from 60 to 250° C. When the tower temperature is at the range from 60 to 180° C., the low temperature dehydration catalysts are used for dehydration. When the tower temperature is at the range from 110 to 250° C., the high temperature dehydration catalysts are used for dehydration.

Preferably, the tower pressure of the catalytic distillation tower is ranging from 8 to 14 bar.

Preferably, the catalytic distillation tower further comprises at least one flash zone at the top of the upper catalytic bed, between the upper catalytic bed and the lower catalytic bed, and at the bottom of the lower catalytic bed; the at least one flash zone is reserved for heat exchange and reflux.

When anhydrous methanol is used as the feeding, the anhydrous methanol is fed to the tower from the feed port located at the top of the upper catalytic bed.

When syngas is used as the feeding, the syngas is firstly converted to a mixture containing DME, methanol and water, and the mixture is fed to the tower from the feed port located at the top of the upper catalytic bed or between the upper and the lower catalytic bed.

When crude methanol is used as the feeding, the crude methanol is fed to the tower from the feed port located at the bottom of the lower catalytic bed or between the upper and the lower catalytic bed.

Compared to the conventional single-bed catalytic distillation tower, the dual-bed catalytic distillation tower of the present invention has advantages of flexible set up depending on various types of feedings such as anhydrous or crude methanol and on different grades of DME to be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the changes of relative volatility of methanol and water at different tower pressures;

FIG. 2 is a liquid-gas phase diagram showing the equilibrium relation between methanol and DME as well as methanol and water at different temperatures under 11 bar;

FIG. 3 is a schematic diagram showing the dual-bed catalytic distillation of the present invention;

FIG. 4 is a schematic diagram showing DME preparation methods using the dual-bed catalytic distillation tower of the present invention;

FIG. 5 is a diagram showing the tower temperature distribution in the catalytic distillation tower of the present invention with anhydrous methanol feeding;

FIG. 6 is a diagram showing the tower temperature distribution in the catalytic distillation tower of the present invention with crude methanol feeding; and

FIG. 7 is a diagram showing the tower temperature distribution in the catalytic distillation tower of the present invention with mixture feeding of DME, methanol and water.

DETAILED DESCRIPTION OF THE INVENTION

The above and other technical features and advantages of the present invention will be described in greater detail with reference to the drawings.

FIG. 2 is a liquid-gas phase diagram showing the equilibrium relation between two substances at different temperatures under 11 bar. The right part of FIG. 2 shows the equilibrium relation between methanol and dimethyl ether (DME). As shown in FIG. 2, when the temperature is close to the saturated liquid temperature of methanol, i.e. 140.5° C., the concentration of methanol increases from the top to bottom of the catalytic distillation tower and the concentration of liquid phase methanol is higher than the concentration of gas phase methanol at the same position of the tower. Therefore, the low temperature dehydration catalyst can be used for dehydration when the dehydration is performed under the conditions shown in Z1 zone in FIG. 2. The left part of FIG. 2 shows the equilibrium relation between methanol and water. When the temperature is close to the saturated liquid temperature of methanol, the concentration of methanol increases from bottom to top of the catalytic distillation tower and the concentration of gas phase methanol is higher than the concentration of liquid phase methanol at the same position of the tower. Therefore, the high temperature dehydration catalyst can be used for dehydration when the dehydration is performed under the conditions shown in Z2 zone in FIG. 2. The saturated liquid temperature of methanol varies with tower pressure. When the tower pressure is lower than 11 bar, the saturated liquid temperature of methanol becomes lower than 140.5° C. On the contrary, the saturated liquid temperature of methanol becomes higher than 140.5° C. when the tower pressure is higher than 11 bar.

Based on the substance properties as described above, the present invention provides a catalytic distillation tower where different catalysts can be placed in different temperature zones to carry out dehydration of methanol efficiently and prevent the catalysts from being damaged due to high temperature. As shown in FIG. 3, the catalytic column in the catalytic distillation tower of the present invention comprises an upper catalytic bed 7 and a lower catalytic bed 11. Three flash zones 8, 9, 10 are respectively located at the top of the upper catalytic bed 7, between the upper catalytic bed 7 and the lower catalytic bed 11, and at the bottom of the lower catalytic bed 11. Three feed ports 1, 2, 3 are respectively located at the flash zones 8, 9, 10. A heat exchanger and/or reflux condenser 12 is located between the upper catalytic bed 7 and the lower catalytic bed 11.

Generally, the temperature of the catalytic distillation tower of the present invention is from 60° C. to 250° C. When the tower temperature is from 60° C. to 180° C., the low temperature dehydration catalyst is used in Z1 as shown in FIG. 2, i.e. the high liquid phase methanol concentration zone. When the tower temperature is from 110° C. to 250° C., the high temperature dehydration catalyst is used in Z2 as shown in FIG. 2, i.e. the high gas phase methanol concentration zone. The transition temperature that needs to be specified for using high temperature dehydration catalysts or low temperature dehydration catalysts is from 110° C. to 180° C.

Preferably, the low temperature dehydration catalyst is used in the upper catalytic bed 7. For example, but not limited to, Amberlyst® 15 acid ion exchange resin is used at tower temperatures ranging from 85 to 110° C. and Amberlyst® 35 acid ion exchange resin is used at tower temperatures ranging from 110 to 135° C. The high temperature dehydration catalyst is used in the lower catalytic bed 11. For example, fluorinated transition metal oxide such as F-alumina, sulfate transition metal oxide such as sulfate zirconium dioxide (SO₄ ²⁻/ZrO₂), β zeolite and HZSM-5 are suitable high temperature dehydration catalysts. The high temperature dehydration catalyst is covered by Teflon to enhance hydrophobic property of the catalyst and prevent the mass transfer resistance caused by the liquid molecular on the surface of the catalyst.

The tower pressure varies depending on the environment. The pressure is usually in the range from 6 bar to 30 bar, preferably from 8 bar to 14 bar. If the catalytic distillation tower is used at high latitudes, 20° C. industrial water is available to condense the DME with a dew point of 25.4° C., the tower pressure can be as low as 6 bar. When the tower pressure is operated at 18 bar, the relative volatility of methanol and water should be considered.

The feeding can be fed into the catalytic distillation tower from different feed ports 1, 2 and 3. When anhydrous methanol is used as the feeding, the anhydrous methanol is usually dehydrated in liquid phase so it is fed to the tower from feed port 1 located at the top of the upper catalytic bed 7. When syngas is used as the feeding, the syngas is firstly converted to a mixture containing dimethyl either, methanol and water, and the mixture is usually fed to the tower from feed port 1 located at the top of the upper catalytic bed 7 or feed port 2 located between the upper catalytic bed 7 and the lower catalytic bed 11. When crude methanol is used as the feeding, the crude methanol is firstly heated to saturated vapor for further reaction so it is usually fed to the tower from feed port 3 located at the bottom of the lower catalytic bed 11 or feed port 2 between the upper catalytic bed 7 and the lower catalytic bed 11. The heat exchanger and/or reflux condenser 12 located between the upper catalytic bed 7 and the lower catalytic bed 11 is used to control the temperatures of the two catalytic beds and simultaneously protect the low temperature dehydration catalysts in the upper catalytic bed 7 from being damaged due to high temperature.

As shown in FIG. 3, the catalytic distillation tower further contains a rectification column 13 at the top of the catalytic column and a stripper column 14 under the catalytic column. After reaction in the catalytic distillation tower, the mixture of methanol and carbon dioxide is discharged from a condenser outlet 4 and DME is discharged and collected from a condenser outlet 5. Unreacted feeding stream flows into the reboiler on the bottom of the tower for separation. After the reaction in the reboiler, water goes out from a reboiler outlet 6 and other substances in the feeding stream flow back to the tower for next reaction.

FIG. 4 shows the application of the dual-bed catalytic distillation tower in the preparation of DME. FIG. 4( a) shows how the dual-bed catalytic distillation tower works in one-step preparation of DME. After the one-step reaction, mixture stream 23 containing DME, methanol, water and carbon dioxide may be fed into the dual-bed catalytic distillation tower from different feed ports 1, 2 and 3 for dehydration to obtain DME. FIG. 4( b) shows how the dual-bed catalytic distillation tower works in two-step preparation of DME. After two-step reaction, the crude methanol stream 22 and anhydrous methanol stream 21 may be fed into the dual-bed catalytic distillation tower from different feed ports 1, 2 and 3 for dehydration to obtain DME.

EXAMPLES

In the following examples, low temperature dehydration rate (An et al., 2004), thermodynamic NRTL-RK equation, the thermodynamic data for high temperature dehydration reaction (Lin et al., 1981; Hayashi, 1982) are used for theoretical calculation of the DME catalytic distillation tower by the simulation software “Aspen Plus” to illustrate the feasibility of the present invention.

The catalytic distillation tower from the top down contains an overhead condenser, rectification column, catalytic column, stripping column and a reboiler at the tower bottom.

The conditions for the conventional single-bed catalytic distillation tower are set as follows. The theoretical plate numbers of the rectification column and the stripping column are respectively set to 7 (including part of or all of the condenser and reboiler). The feed containing methanol mixture is fed to the catalytic distillation tower at 30° C., and performs heat exchange with the hot water leaving the bottom of the catalytic distillation tower. The feed temperature is about 40° C. after the heat exchange step. The amount of catalyst and the tower height are adjusted, i.e. changing the plate number of the catalytic column and the tolerated flow rate for liquid in the catalytic column (24 inch height/plate) to be 24.4 m/s. Afterwards, the reflux ratio and D/F are adjusted so the concentration of DME and water produced from the catalytic distillation tower are respectively 99.9 wt %.

In the dual-bed catalytic distillation tower, the catalytic column includes an upper catalytic bed filled with low temperature dehydration catalysts and a lower catalytic bed filled with high temperature dehydration catalysts. The position in the tower having a temperature of 135° C. is set as the demarcation point for the upper and lower catalytic beds. The plate number of the catalytic column, the reflux ratio and D/F are adjusted so the concentration of DME and water produced from the catalytic distillation tower are respectively 99.9 wt %. When the fuel grade DME (93 wt % of DME and 7 wt % of methanol) is desired under the same catalytic distillation design, the amount of catalysts is excessive. In this situation, the feeding is increased to lower the conversion of methanol, such that the fuel grade DME is obtained from the top of the tower, and the concentration of the water obtained from the bottom of the catalytic distillation tower is still 99.9 wt %.

The conditions of the present dual-bed catalytic distillation tower are assumed as below. The unit volume of the upper catalytic bed equals 0.6 unit volume of the theoretical plate and the unit volume of the lower catalytic bed equals 0.85 unit volume of the theoretical plate (referring to U.S. Pat. No. 6,045,762 and US 2007/0,095,646). The dehydration rate of the hydrophobic and strong acidic catalysts is five times more than the rate disclosed in the literature of Lin et al., 1981.

Comparative Example Anhydrous Methanol as Feeding (Prior Art)

The simulation for the production of DME in US 2007/0,066,855 is indicated below.

Tower pressure 12 bar Pressure drop 0.7 bar Reflux ratio  2 D/F weight ratio  0.71773 Theoretical plates 30 Feeding plate At plate number 8 Catalyst column From plates number 10 to 18 Catalyst amount 140 m³ Methanol fed 5013 tpd (99.88 weight %) DME produced 3596 tpd (92.35 weight %) Temperature on the top of the tower 56.1° C. Temperature on the bottom of the tower 18.9° C.

The result shows that the DME production rate is 3598 tpd (99.61 weight %) and the temperatures at the top and bottom of the tower are respectively 52° C. and 190° C. Although the tower temperature distribution is not disclosed in the prior art, the temperature is assumed having 4° C. inaccuracy compared to the simulated tower temperature.

The result shows that the temperatures at the bottom two plates, i.e. plates 17 and 18, are respectively 151.5° C. and 156.4° C. which exceed the limit of temperature resistance of acid ion exchange resin catalysts such as Amberlyst® 35 having temperature resistance at 140° C. Therefore, the low temperature dehydration catalysts cannot be used in the conventional catalytic distillation tower under certain tower pressure such as at the pressure ranging from 8 to 12 bar.

Example 1 Anhydrous Methanol as Feeding

The simulation for the production of DME in the dual-bed catalytic distillation tower of the present invention and in the conventional single-bed catalytic distillation tower is indicated below.

Tower pressure   11 bar Pressure drop 0.025 bar Initial reflux ratio 4 D/F molar ratio 0.4997 Feeding plate At plate number 7

The results are shown in Table 1, Table 2 and FIG. 5.

TABLE 1 Stream composition during the catalytic distillation process Catalytic Column Single-bed* Dual-bed Dual-bed DME grade Chemical Chemical Fuel Stream*** 1 5 6 1 5 6 2 5 6 Water, kg/hr 6 0 1689 6 0 1689 7 0.1 1791 Methanol, kg/hr 5994 4.3 1.7 5994 4.3 1.7 6693 343 1.8 DME, kg/hr 0 4305 0 0 4305 0 0 4564 0 Total, kg/hr 6000 4309 1691 6000 4309 1691 6700 4907 1793 *Conventional method, the low temperature dehydration catalyst does not work at high temperature. ***Stream composition at the feed port 1 and 2, condenser output 5 and reboiler output 6 shown in FIG. 3.

TABLE 2 The results for the preparation of DME using single-bed and dual-bed catalytic distillation tower Catalytic Column Single-bed* Dual-bed Dual-bed DME grade Chemical Chemical Fuel D/F, mole 0.4993 0.4993 0.5246 Reflux ratio 1.728 1.768 1.106 Feeding plate 7 7 7 number Plate number Condenser 1 1 1 Rectification 2-7 2-7 2-7 column Upper catalytic bed  8-20  8-16  8-16 Lower catalytic bed N/A 17-24 17-24 Stripping column 21-26 25-30 25-30 Reboiler 27 31 31 Low temperature 19.7 13.7 13.7 catalyst, m³ High temperature N/A 17.2 17.2 catalyst, m³ Tower temperature, Rectification 48.3-108  48.3-108   52-110 column Upper catalytic bed 108-178 108-134 110-135 Lower catalytic bed N/A 139-177 139-177 Stripping column 182-187 183-187 183-187 Pressure at tower 11 11 11 top, bar *Conventional method, the low temperature dehydration catalyst does not work at high temperature.

As shown in Tables 2 and 4, more plates are needed in the dual-bed catalytic distillation tower compared to the single-bed catalytic distillation tower in order to increase methanol conversion rate. This is because the reaction rate of the high temperature dehydration catalyst is lower than the reaction rate of the low temperature dehydration catalyst, so the amount of high temperature dehydration catalyst is more than the amount of low temperature dehydration catalyst. In the present invention, high temperature dehydration catalysts (from plates 17 to 24) are used to replace part of the low temperature dehydration catalysts (from plates 17 to 20) in the conventional catalytic distillation tower, so the temperature increases moderately at the lower catalytic bed. Therefore, the temperature at the border between the upper catalytic bed and the lower catalytic bed would not be too high and damage the low temperature dehydration catalysts in the upper catalytic bed.

Example 2 Crude Methanol as the Feeding

The feeding comes from the process for preparing methanol from syngas (with reference to U.S. Pat. No. 5,750,799). The produced methanol containing 10-20 mole % of water is fed to the catalytic distillation tower of the present invention directly for dehydration without further purification. The simulation for the production of DME in the dual-bed catalytic distillation tower of the present invention and in the conventional single-bed distillation tower is indicated below.

Tower pressure   11 bar Pressure drop 0.025 bar Initial reflux ratio 4 D/F molar ratio 0.4997

The results of using dual-bed catalytic distillation tower of the present invention and using conventional single-bed catalytic distillation tower are shown in Table 3, Table 4 and FIG. 6.

TABLE 3 Stream composition during the catalytic distillation process Catalytic Column Single-bed* Dual-bed Dual-bed Single-bed** DME grade Chemical Chemical Chemical Chemical Feeding plate number**** 7 7 9 16 Stream*** feeding 5 6 5 6 5 6 5 6 water, kg/hr 666 0 2349 0 2349 0.08 2336 0 2349 methanol, kg/hr 5994 4.3 2.4 4.3 2.4 28.3 26.6 4.3 2.4 DME, kg/hr 0 4304 0 4304 0 4270 0 4304 0 Total, kg/hr 6660 4309 2351 4309 2351 4298 2362 4309 2351 *Conventional method, the low temperature dehydration catalyst does not work at high temperature. **Conventional method of using high temperature catalysts. ***Stream composition at condenser output 5 and reboiler output 6 shown in FIG. 3. ****Feeing at plate number 7 means feeding from feed port 1, feeding at plate number 9 means feeding from feed port 2, feeding at plate number 16 means feeding from feed port 3.

TABLE 4 The results for the preparation of DME using single-bed and dual-bed catalytic distillation tower with crude methanol Catalytic Column Single-bed* Dual-bed Dual-bed Single-bed** DME grade Chemical Chemical Chemical Chemical Feeding plate 7 7 9 16 number D/F, mole 0.4176 0.4176 0.4176 0.4176 Reflux ratio 1.228 1.28 2.86 13.1 Plate number Condenser 1 1 1 1 Rectification 2-7 2-7 2-6 2-7 Column Upper catalytic bed  8-12 8-9 7-8 N/A Lower catalytic bed N/A 10-17 10-17  8-15 Stripping column 13-18 18-23 18-23 16-21 Reboiler 19 24 24 22 Low temperature 11.6 4.66 4.66 N/A catalyst, m³ High temperature N/A 26.4 26.4 22.6 catalyst, m³ Tower temperature, Rectification 48.3-100  48.3-101  48.6-122  48.3-154  column Upper catalytic bed 120-170 120-131 125-128 N/A Lower catalytic bed N/A 136-171 136-160 164-174 Stripping column 177-186 178-186 168-186 172-186 Pressure at tower 11 11 11 11 top, bar *Conventional method, the low temperature dehydration catalyst does not work at high temperature. **Conventional method, use high temperature dehydration catalyst.

When the methanol is fed from feed port 1, the liquid flow amount is more than the liquid flow amount in Example 1 because the methanol feeding contains water. Therefore, the tower diameter in this example is larger than the tower diameter in Example 1. The temperature distribution in the tower from theoretical calculation is similar to the tower temperature distribution shown in FIG. 5. The temperature of the lower catalytic bed in the dual-bed catalytic distillation system is lower than the temperature in the single-bed distillation system at the same position. Accordingly, the low temperature dehydration catalyst is better protected in dual-bed catalytic distillation system.

When the upper catalytic bed is moved a plate upper and methanol is fed from feed port 2, the reflux rate is raised to increase the conversion rate of methanol. The concentrations of DME at the tower top and water at the tower bottom respectively decrease to 99.3 wt % and 98.9 wt %.

When methanol is fed from feed port 3, only high temperature dehydration catalysts can be used in the single-bed distillation system because low temperature dehydration catalysts will be damaged when the column temperature is higher than 160° C.

Example 3 Mixture of DME, Methanol and Water as the Feeding

The feeding comes from the process for preparing methanol from syngas (with reference to U.S. Pat. No. 5,908,963). After separation, the concentration of carbon dioxide, DME, methanol and water in the product from the reactor are respectively 2.8 wt %, 49.7 wt %, 31.0 wt % and 16.5 wt %. The mixture can be fed at the top catalytic column or between the upper and lower catalytic beds. The reflux ratios are calculated to be 0.378 and 1.22. The results are shown in Table 5, Table 6 and FIG. 7.

TABLE 5 Stream composition during the catalytic distillation process Catalytic Column Single-bed* Dual-bed Dual-bed DME grade Chemical Chemical Chemical Feeding plate number**** 7 7 9 Stream*** 1 5 6 1 5 6 2 5 6 Water, kg/hr 3190 0 4870 3190 0 4870 3190 0 4864 Methanol, kg/hr 5994 13.6 4.9 5994 13.7 4.9 5994 25 15 DME, kg/hr 9610 13905 0 9610 13905 0 9610 13890 0 Total, kg/hr 18794 13919 4875 18794 13919 4875 18794 13915 4879 *Conventional method, the low temperature dehydration catalyst does not work at high temperature. ***Stream composition at the feed port 1 and 2, condenser output 5 and reboiler output 6 shown in FIG. 3. ****Feeing at plate number 7 means feeding from feed port 1, feeding at plate number 9 means feeding from feed port 2.

TABLE 6 The results for the preparation of DME using single-bed and dual-bed catalytic distillation tower Catalytic column Single-bed* Dual-bed Dual-bed DME grade Chemical Chemical Chemical Feeding plate 7 7 9 number D/F, mole 0.5278 0.5278 0.5278 Reflux ratio 0.354 0.378 1.22 Plate number Condenser 1 1 1 Rectification 2-7 2-7 2-6 Column Upper catalytic bed  8-12 8-9 7-8 Lower catalytic bed N/A 10-14 10-14 Stripping column 13-18 15-20 15-20 Reboiler 19 21 21 Low temperature 16.4 6.6 6.6 catalyst, m³ High temperature N/A 23.3 23.3 catalyst, m³ Tower temperature, Rectification 48.3-64   48.3-66   48.3-96   column Upper catalytic bed  89-173  94-125 101-104 Lower catalytic bed N/A 140-172 134-167 Stripping column 179-186 179-186 174-186 Pressure at tower 11 11 11 top, bar *Conventional method, the low temperature dehydration catalyst does not work at high temperature.

FIG. 7 shows the temperature changes in the catalytic column. Feeding the mixture at the feed port 2 between the upper and lower catalytic beds alleviates rapid temperature changes in the catalytic column and simultaneously protects the catalysts in the low temperature catalytic bed from being damaged. As shown in FIG. 3, heat exchange/reflux may be carried out at the flash zone 9 to control the temperature at the catalytic column and protect the low temperature dehydration catalyst from being damaged.

Example 4 Mixture of DME, Methanol, Carbon Dioxide and Water as the Feeding

The feeding comes from the process for preparing methanol from syngas (with reference to U.S. Pat. No. 5,908,963). After separation, the concentration of carbon dioxide, DME, methanol and water in the product from the reactor are respectively 2.8 wt %, 49.7 wt %, 31.0 wt % and 16.5 wt %. The carbon dioxide can-not be liquefied because of the limitation of the tower pressure. Thus, an overhead condenser is equipped on the top of the catalytic distillation tower. The results are shown in Table 7. The obtained DME contains 1.36 wt % of carbon dioxide, so it will flow back to the catalytic distillation tower for purification. The carbon dioxide and DME in gas phase are respectively 25 wt % and 75 wt %. They also need to be re-purified in the catalytic distillation tower.

TABLE 7 The results for the preparation using dual-bed catalytic distillation tower Catalytic Dual-bed column Feeding plate 7 number**** Stream*** 1 4 5 6 Water 3101 0 0.6 4733 Methanol 5826 0.1 12.7 4.7 Dimethyl ether 9341 1038 12479 0 Carbon dioxide 526 354 172 0 Total, kg/hr 18794 1392 12664 4738 Results D/F, mole 0.538 Gas/tower top 0.1 distillate ratio Reflux ratio 0.343 Plate number Condenser 1 Rectification 2-7 Column Upper catalytic 8-9 bed Lower catalytic 10-13 bed Stripping 14-19 column Reboiler 20 Low 6.9 temperature catalyst, m³ High 19.4 temperature catalyst, m³ Tower temperature, Rectification 41-68 column Upper catalytic  80-112 bed Lower catalytic 143-185 bed Stripping 190-193 column Pressure at 13 tower top, bar ***Stream composition at the feed port 1 and 2, condenser output 5 and reboiler output 6 shown in FIG. 3. ****Feeing at plate number 7 means feeding from feed port 1.

What stated above is only preferred embodiments of the present invention, which is illustrative only and not restrictive. Changes, modifications, or the equivalents may be made by those skilled in the art without departing from the spirits and scope of the present invention as defined by the claims, but will fall within the scope of protection of the present invention. 

What is claimed is:
 1. A dual-bed catalytic distillation tower comprising a catalytic column comprising an upper catalytic bed and a lower catalytic bed, wherein at least one feed port is located at the top of said upper catalytic bed, between the upper and lower catalytic beds, and at a bottom of said lower catalytic bed; wherein said upper catalytic bed is filled with low temperature dehydration catalysts and said lower catalytic bed is filled with high temperature dehydration catalysts, wherein the temperature of said upper catalytic bed ranges from 85° C. to 135° C., and the temperature of said lower catalytic bed ranges from 135° C. to 250° C.
 2. The dual-bed catalytic distillation tower according to claim 1, wherein the low temperature dehydration catalyst is Amberlyst® 15 acid ion exchange resin or Amberlyst® 35 acid ion exchange resin.
 3. The dual-bed catalytic distillation tower according to claim 1, wherein the high temperature dehydration catalyst is selected from fluorinated transition metal oxide, sulfate transition metal oxide, β zeolite and HZSM-5.
 4. The dual-bed catalytic distillation tower according to claim 1, wherein the high temperature dehydration catalysts are covered by Teflon.
 5. A method for preparing dimethyl ether (DME) by using the dual-bed catalytic distillation tower according to any one of claims 1 to 4, wherein a feed containing methanol is fed to said catalytic distillation tower at the top of said upper catalytic bed, between said upper and lower catalytic beds, or said bottom of said lower catalytic bed at a tower pressure ranging from 6 to 30 bar for dehydration to obtain DME.
 6. The method for preparing DME according to claim 5, wherein the tower pressure is from 8 to 14 bar.
 7. The method for preparing DME according to claim 5, wherein at least one flash zone at said top of the upper catalytic bed, between said upper catalytic bed and the lower catalytic bed, and at the bottom of said lower catalytic bed is reserved for heat exchange and reflux.
 8. The method for preparing DME according to claim 5, wherein said feed is anhydrous methanol and is fed at the top of said upper catalytic bed.
 9. The method for preparing DME according to claim 5, wherein said feed is a mixture of DME, methanol and water obtained from syngas and is fed between said upper and lower catalytic beds.
 10. The method for preparing DME according to claim 5, wherein said feed is crude methanol and is fed at said bottom of said lower catalytic bed or between said upper and lower catalytic beds. 